bisect: no longer try to clean up left-over `.git/head-name` files

bisect: no longer try to clean up left-over `.git/head-name` files

As per the code comment, the `.git/head-name` files were cleaned up for
backwards-compatibility: an old version of `git bisect` could have left
them behind.

Now, just how old would such a version be? As of 0f497e75f05 (Eliminate
confusing "won't bisect on seeked tree" failure, 2008-02-23), `git
bisect` does not write that file anymore. Which corresponds to Git
v1.5.4.4.

Even if the likelihood is non-nil that there might still be users out
there who use such an old version to start a bisection, but then decide
to continue bisecting with a current Git version, it is highly
improbable.

So let's remove that code, at long last.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>